Method and apparatus for sealing abandoned oil and gas wells

ABSTRACT

Apparatus and method for forming a solid sealing plug of bismuth-tin alloy material within a well casing for sealing oil or gas wells. Solid alloy material is positioned within a heating tool and lowered to a position within the well casing where the seal is to be formed. The heating tool is heated to liquefy the alloy material and the alloy material then runs out of the heating tool and solidifies on top of a cement plug previously formed within the well casing. A cement slurry or other fluid material may subsequently be deposited on top of the liquefied alloy material to enhance the sealing of the alloy plug, to form a barrier to subsequent creep of the alloy plug when the alloy solidifies and to counteract any pressure acting vertically on the bottom of the plug.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of provisional patent application Ser. No. 61/052,999 filed May 13, 2008.

INTRODUCTION

This invention relates to a method and apparatus for sealing abandoned oil and gas wells and, more particularly, to sealing abandoned oil and gas well utilising an eutectic alloy which expands upon passage from the liquid to solid state.

BACKGROUND OF THE INVENTION

When oil and gas wells are shut in or abandoned, a regulatory framework exists which mandates the procedures and technology required to properly shut in or abandon the well. This is required to prevent so far as is possible the leakage of gas from the underground formations to the surface. Such leakage can have adverse consequences from unpleasant smells and site contamination to creating a possibly latent explosive condition or the release of a toxic gas such as hydrogen sulfide.

Heretofore, following the addition of cement to the production and surface casings following abandonment, a steel cap was sealingly welded to the top of the outermost casing. Such a cap forms a “last barrier” to the seepage of any gas through the cement within the casing. The steel cap is usually placed on the top of the casing beneath the ground surface a certain distance, usually six feet or so, to prevent the casing from being contacted by farm implements and other earth moving or working equipment when agricultural land is being worked following well abandonment.

Because of significant real estate developments caused by increasing population in urban areas, there may be a number of previously abandoned wells in proximity to areas being developed. Many operations may occur underground at depths considerably below the six feet level and the possibility of underground machinery being used which can contact and damage the casing and cap is much more likely now than years ago. It has also been found that in many abandoned wells, gas has migrated over time through the cement upwardly within the casing and a pressure head is formed directly below the welded steel cap. If the casing or cap is damaged, this trapped gas may escape giving rise to the aforementioned significant problems.

SUMMARY OF THE INVENTION

According to one aspect of the invention, there is provided a method for sealing an oil or gas well comprising positioning a melted bismuth/tin alloy material within a casing to form a plug within said casing when said liquefied alloy material solidifies within said casing.

According to a further aspect of the invention, there is provided apparatus to allow a molten bismuth-tin alloy material to be positioned as a plug within an oil or gas well, said apparatus comprising means to add molten eutectic material to said casing at a predetermined location within said casing so as to form a solid plug within said casing when said liquefied alloy material cools and means to add a force balancing material to said casing on the top of said molten material.

According to yet a further aspect of the invention, there is provided a method of sealing an abandoned oil or gas well comprising removing the cap on the top of the outer casing of said well, lowering and positioning a melted bismuth/tin alloy material within said casing to form a plug within said casing when said liquefied alloy material solidifies within said casing, positioning a force balancing material on the top of said melted bismuth/tin alloy material and allowing said melted bismuth/tin alloy to cool. a method for sealing an oil or gas well comprising positioning a predetermined quantity of melted bismuth/tin alloy material within a casing to form a plug wholly within said casing when said liquified alloy material cools and solidifies within said casing.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Specific embodiments of the invention will now be described, by way of example only, with the use of drawings in which:

FIG. 1A is a diagrammatic side and cutaway view of the solidified alloy plug within the alloy heater which has been lowered into the casing of an oil or gas well and which is attached to the surface controlled wireline;

FIG. 1B is a diagrammatic view similar to FIG. 1A but showing the alloy heater in operation with the alloy in a molten state;

FIG. 1C is a diagrammatic view similar to FIGS. 1A and 1B but illustrating the alloy heater being withdrawn from the well and leaving the molten alloy within the well;

FIG. 1D is a diagrammatic side view of the alloy plug left in place within the well in its cooled and solid state;

FIG. 2 is a diagrammatic view of the components utilised to form the alloy plug within the well casing; and

FIG. 3 is an enlarged side view of the heating tool used to melt the bismuth-tin alloy and to carry the cement slurry which is deposited on the alloy material.

DESCRIPTION OF SPECIFIC EMBODIMENT

Referring now to the drawings, an oil or gas well is shown generally in enlarged form at 100 in FIG. 1. It comprises production casing illustrated generally at 101 which is cemented in and surrounded with cement 102. A solid cement plug 103 is illustrated in place at the position of interest within the production casing 101. The setting and formation of the cement plug 103 is well known in the art.

The components generally used for setting and forming the bismuth-tin alloy plug are best illustrated in FIG. 2. Such components comprise a power control unit 104 located on the surface 114 which also serves as the source of input power, generally 480 volt three-phase alternating current which is subsequently rectified to adjustable voltage DC current for transmission to a heating tool 111. The required power connections 105 are connected to a wireline spool 110 and extend from the spool 110 downhole by way of wireline cables 113 to an attachment 115 of the heating tool 111 which is diagrammatically illustrated in position within the production casing 101 with the cement or bridge plug 103 illustrated as being in place. A lubricator 112 to maintain a pressure seal may also be required as the cables 113 and heating tool 111 move up and down within the well casing 101.

The longitudinal and circular heating tool 111 is illustrated in greater detail in FIG. 3. The power carrying cables 113 extend through an attachment point 115 on the tool 111 and terminate at an instrument pod 120. The instrument pod 120 contains the necessary electronics to monitor downhole performance of the heating tool 111 and also provides for power transfer from the cables 113 to the circumferential alloy heating heaters 121 to be described in greater detail hereafter. The heating tool 111 contains a first circular cavity 122 (see also FIG. 1A) adapted to hold the bismuth-tin alloy in solid form and to allow the alloy to melt and run from the circular cavity 122. The heating tool 111 further contains a second circular cavity 123 which is generally concentric to and of identical internal configuration to first circular cavity 122 although the length of second cavity 123 may be increased or decreased in order to hold a required amount of cement slurry 124. A loading port 130 is also provided to allow the loading of alloy billets (not illustrated) or of liquid alloy material as 131 as well as a force balancing material such as a cement slurry 124 although other materials may be water, sand, gravel or other suitable and fluid materials.

The pressure sealing material that is preferred in the present operation is a bismuth-tin alloy mixture having 58% by weight bismuth and 42% by weight tin alloy. Bismuth is the essential ingredient inasmuch as it is non-toxic and exhibits the valuable property that it expands volumetrically upon solidification from the liquid phase. This expansion causes an effective fluid seal when placed within a well casing in molten form. Tin is also non-toxic, hence the mixture can be tolerated in direct contact with fresh groundwater which is a desirable characteristic for a well plugging material. Whereas any composition of bismuth-tin alloy could be used, the most favorable is the aforementioned 58/42 composition because this mixture is a eutectic mixture melting and solidifying at 137 deg. C. This is the minimum temperature at which a bismuth-tin alloy can exist entirely as a liquid and, therefore, facilitates the process of in situ melting and placement of the alloy plug.

The use of bismuth material to form the sealing plug 132 illustrated in FIG. 1C is desirable for the principal reason that bismuth expands as it solidifies. This is advantageous since while the alloy is in liquid form, it will fill and run into interstices that might be used as eventual passageways for fugitive gas transmission and, as the alloy cools and solidifies, it expands to fill the constrained volume of the well casing and therefore forms a far better seal than that of a material that may contract or remain at the same volume upon cooling.

The size of the plug 132 which is required will generally be known in order to utilise the correct quantity of alloy. A rule of thumb generally used in the art is that the plug 132 will be approximately three times in length as compared to the diameter of the casing 101. Clearly, this dimension will vary particularly if the wellhole is deep and pressures downhole are high in which event a plug of greater lengthwise dimension would be desirable. But because the plug 132 is wholly within the casing 101, the magnitude of alloy required will be far more accurate than when the alloy material is being used to seal a geological formation outside the wellbore by way of perforations in the casing.

Operation

In operation, there are several techniques that may be used to set up the heating tool 111 for downhole operation. Preferably, a cap 133 (FIG. 4) is attached to the bottom of heating tool 111. Cylindrical bismuth-tin billets (not illustrated) may be added through the loading port 130 and positioned one on top of the other within the billet magazine 134 with the lowermost billet being in contact with the cap 133. Thereafter, heat is applied to the heaters 121 which surround the circular billets in order to melt the billets within the cavity 122. The heating is preferably of resistive or inductive nature but surface heating of the billets may conveniently be performed using other heating techniques. The heating is terminated following the melting of the billets and the bismuth-tin alloy solidifies with the cylindrical heating cavity 122 as also seen in FIG. 1A. The cap 133 is then removed and the tool 111 is ready for downhole operation.

When it is desired to plug a well, the tool 111 is attached to the power and wireline cables 113 which lower and raise the heating tool 111 within the casing 101. At this juncture, a predetermined quantity of force balancing material such as cement slurry 124 (FIG. 3) is added to the tool 111 through the loading port 130. The cement slurry is positioned on top of the solid alloy material 131 and is intended to remain in fluid form until it exits the heating tool 111 when the alloy plug is being formed. The quantity of cement slurry or other force balancing material is dependent upon the pressure which is acting on the plug but the quantity required need not be highly accurate since the cost of the force balancing material is not great.

The heating tool 111 is lowered into the wellhole as best seen in FIGS. 1A and 2 until it contacts the bridge or cement plug 103. Heat is then applied to the solid alloy material by the heaters 121 of the alloy heater 111 until its melting point is reached at which point gravity will tend to move the liquid alloy out of the cavity 122 (FIG. 1A) of the heating tool 111 as seen in FIG. 1B. The weight of the heating tool 111 is monitored and as the alloy 116 runs out of the heating tool 111, the tool 111 is raised within the casing away from cement plug 103 and the liquid alloy forms a plug 132 within the casing 101 as best seen in FIG. 1C. The fluid force balancing material (not shown) such as the liquid cement slurry will follow the liquefied alloy out of the heating tool 111 and forms a counterforce type layer on top of the alloy plug 132. It will be observed that the action of the force balancing material on the molten alloy will have at least two interesting characteristics. First, since the alloy is at a temperature greater than the force balancing material, the contact between the force balancing material and the top of the liquefied alloy plug 132 will cool the top of the alloy faster than at the bottom. Thus, expansion of the alloy at the top of the plug 132 will occur before the alloy expands as it cools in the lower portions of the plug 132. This enhances the seal at the top of the alloy plug 132 and forms resistance to any subsequent creep in the plug 132 caused by well pressure. Second, as the force balancing material solidifies on top of the plug 132, it also serves as a barrier to any subsequent creep of the alloy material in the plug 132 over time.

Following the release of the alloy material and the force balancing material, the heating tool 111 is withdrawn from the casing 101 by use of the wireline cables 113 and the heating operation is terminated as seen in FIG. 1C. A significantly improved plug 132 (FIG. 1B) is formed in the casing 101 which will reduce or eliminate the migration of gases to the surface though the well casing 101.

While the use of alloy billets has been described, it is envisioned that molten alloy material may be added through the loading port 130 rather than in solid billet form which liquefied alloy material will then run down within the heating cavities to the temporary cap 133. The alloy material is then allowed to cool and the cap 133 is removed as described earlier.

While the force balancing material has been described as being added to the heating tool and subsequently released by the tool upon the exit of the alloy plug material, it is also envisioned that the force balancing material could be added to the wellhole in other manners such as simply lowering an automatically or manually opening bucket or other container. Sand, for example, could simply be poured down the wellhole following the installation of the alloy plug.

Although the force balancing material described herein is preferably a cement slurry, other materials such as sand, gravel, water or other suitable materials could conveniently be used.

Many modifications will readily occur to those skilled in the art to which the invention relates and the particular embodiments described herein should be taken as illustrative of the invention only and not as limiting its scope as defined in accordance with the accompanying claims. 

1. A method for sealing an oil or gas well comprising positioning a melted bismuth/tin alloy material within a casing to form a plug within said casing when said liquefied alloy material solidifies within said casing.
 2. A method as in claim 1 and further positioning a slurry cement material on the top of said melted bismuth/tin alloy material and allowing said melted bismuth/tin alloy to cool.
 3. A method as in claim 2 wherein said bismuth-tin alloy material is melted by the application of heat with a heating tool downhole in said oil and/or gas well.
 4. A method as in claim 3 wherein said bismuth-tin alloy material is positioned within said heating tool in a first solid form prior to being melted.
 5. A method as in claim 4 wherein said heating tool is lowered within a well casing to an existing cement plug within said well casing with said bismuth-tin alloy in solid form and said force balancing material is positioned on top of said bismuth-tin alloy.
 6. A method as in claim 5 wherein said bismuth-tin alloy is heated following the lowering of said heating tool within said well casing, said liquefied alloy material exiting said heating tool and said force balancing material exiting said heating tool following said alloy material.
 7. A method as in claim 1 wherein a cap is initially removed from the top of the outer one of said casing to allow access to said casing.
 8. Apparatus to allow a molten bismuth-tin alloy material to be positioned as a plug within an oil or gas well, said apparatus comprising means to add molten eutectic material to said casing at a predetermined location within said casing so as to form a solid plug within said casing when said liquefied alloy material cools and means to add a force balancing material to said casing on the top of said molten material.
 8. Apparatus as in claim 7 wherein said molten eutectic material is a bismuth-tin alloy material.
 9. Apparatus as in claim 8 wherein said means to add said bismuth-tin alloy material is an alloy heater with a first cylindrical cavity to hold said bismuth-tin alloy material in solid form and said means to add a force balancing material is a second cylindrical cavity to hold said force balancing material in a fluid form.
 10. Apparatus as in claim 9 wherein said heating tool has a heating chamber which allows said bismuth-tin alloy in solid form to be liquefied within said heating tool.
 11. Apparatus as in claim 10 wherein said means to add molten alloy material to said casing further includes power cables extending to said heating tool and a wireline to allow said heating tool to be raised and lowered within said well casing.
 12. Method of sealing an abandoned oil or gas well comprising removing the cap on the top of the outer casing of said well, lowering and positioning a melted bismuth/tin alloy material within said casing to form a plug within said casing when said liquefied alloy material solidifies within said casing, positioning a force balancing material on the top of said melted bismuth/tin alloy material and allowing said melted bismuth/tin alloy to cool. 